The invention relates to an apparatus for measuring the viscosity of plastic materials, particularly of polymer melts in accordance with the principle of determining the pressure drop of the material when flowing through a capillary and to a measuring method for determining the flow curve of such plastic material.
Such an apparatus and such a method are known for example from DE 42 20 157.
The quality control in the production of polymers comprises a number of examinations of different properties, such as the Theological properties, the determination of foreign particles in the melt, the determination of the chemical composition or, respectively, of the additives in a polymer.
In the past, the various properties were determined by separate measuring apparatus and methods. Today, however, the trend is to determine a multitude of properties in a single measuring apparatus or method.
Today, laboratory extrusion apparatus are in use in which at the same time or in successive steps:
a polymer is melted,
rheological properties of the polymer, such as the viscosity, the melt flow index (MFI), or the melt volume index (MVI) are continuously determined.
a foil is produced which is continuously tested optically for the presence of foreign matter, such as gels, specks, black specks, or
the foil is further utilized to continuously determine the content of certain additives by means of infrared spectroscopy.
The determination of the viscosity is therefore a partial task, which must be incorporated optimally into the total operating process of a laboratory extrusion apparatus.
With continuously operating so-called on-line rheometers, a distinction is to be made on one hand between so-called side stream rheometers which are fed by a side stream out of the extrusion apparatus and which discharge the material flow out of the capillary onto the ground and, on the other hand, the so-called in-line rheometers which are arranged in the mainstream of the melt between the worm kneader and the nozzle and wherein the melt, after passing through the rheometer, is returned to the main stream and discharged through the nozzle for further processing.
In the state of the art, the viscosity of a plastic material is determined, for example, by measuring the pressure loss upon flowing through a capillary of a predetermined dimension and length.
In that case, pre-conditions for the determination of the viscosity are:
the knowledge of the accurate amount of the material flowing through the capillary; for the examination of polymer melts, this amount is preferably defined by the use of a gear pump which is driven with a predetermined speed so as to provide a uniform flow volume of viscous material,
the knowledge of the geometry of the measuring capillary; if a slot capillary is used, the width, the height, and the length of the capillary are known; if a round capillary is used, the diameter and the length are known,
the adjustment of an accurate temperature of the melt and the area around the measuring capillary;
the knowledge of the specific weight of the substance to be measured;
the measurement of the pressure difference between inlet and outlet of a round capillary that is, respectively, the pressure difference between two points of a slot capillary. With this measurement value, a measuring point of the viscosity with a predetermined shear velocity can be obtained utilizing known mathematical equations. However, with polymer melts, the non-Newton or structure-viscous behavior is a big problem, that is, at different shear speeds, different viscosity values are obtained.
For determining the behavior of melts in the various, very different processes employed in the industrial plastic processing industry, very different shear speeds have to be covered, that is shear speeds in the range of about 1:104.
With a capillary and a melt pump, normal ranges of 1:10 and maximal ranges of 1:100 of the shear speed can be covered. This, however, is possible only by varying the speed of the melt pump in a very wide range.
Therefore, for some time already, arrangements of multiple capillaries are used in order to increase the range of the shear speeds that can be measured.
In the known arrangement according to DE 42 20 157 A1, for example, slot capillaries with stepwise decreasing heights are utilized. In this way, the range of the shear speeds that can be measured is increased by the power of ten. In order to cover the wide range of shear speeds, additionally the speed of the melt pump must be changed substantially. When used as an in-line measuring apparatus, the travel time of the material in the relatively long nozzles is substantial whereby the material can be damaged.
DE 42 36 407 discloses an apparatus for measuring the viscosity of viscous materials particularly of polymer melts utilizing the principle of determining the pressure drop experienced by a mass flowing through a capillary with predetermined cross-section and predetermined length for use in a laboratory, particularly for the continuous measurement in a manufacturing plant especially with an integrated quality control. It includes a controllable melt pump for generating a predetermined melt flow and is installed in a heatable device and several capillaries are installed in the apparatus for accommodating a large range of flow speeds.
Ind. Lab. (1974) 40, pages 1467-1468 discloses a four channel viscosimeter, wherein melt can be supplied at the same time to four exchangeable capillaries. The mass flows in the capillaries act on a metal strip. The deformation of the metal strip is sensed and analyzed at the same time in all four capillaries.
In another known apparatus according to GB-A-2 271 856, several round capillaries are slideably so arranged that melt can be supplied to them from a melt pump in succession. It is a disadvantage in this arrangement that a uniform heating of the slideable capillaries is problematic. Especially, the use of a single mass pressure sensor for the complete pressure range to be measured is problematic, since, in this case, the measuring accuracy is insufficient in the very low pressure ranges with capillaries of large cross-section and very low shear velocities.
Furthermore, U.S. Pat. No. 4,677,844 discloses an apparatus for measuring the viscosity of viscous masses on the basis of the principle of determining the pressure drop of the mass when flowing through a capillary with a defined cross-section and defined length, wherein four capillaries are in communication with a cylinder and these capillaries are placed in communication with the surrounding atmosphere by operating a control mechanism. When a piston in the cylinder is operated, the mass contained in the cylinder is pressed through a selected capillary. That measurement step is repeated for each capillary.
In another arrangement, several capillaries are supplied with melt by several melt streams of a multiple gear pump. This is consequently a multiple arrangement of individual melt pumps each with an associated capillary.
Such an arrangement is disclosed for example in U.S. Pat. No. 4,425,790. Herein, three or four capillaries are arranged adjacent one another in series. From one capillary to the next, the size of the flow passages of the capillaries increases wherein the ratio of the capillary length to the capillary cross-section remains essentially the same for all capillaries. A heated polymer melt is pressed by means of a pressure pump through the capillaries with a constant volume flow rate. Sensors determine the pressure and temperature in each capillary. In this way, it is possible to determine the polymer viscosity in each capillary for different shear velocities.
In this way, large shear speed ranges can be measured with a single apparatus. However, the consumption of melt is doubled or tripled which results in increased new material expenses and causes re-granulation. In contrast, it is desirable, particularly in the continuous quality control, to minimize the amount of testing material being wasted.
In other known systems as they are disclosed, for example in DE 44 42 172 C2, FIG. 2, the melt is not discarded after passing through the capillary, but, is returned into the melt stream of the extruder. Although this method avoids, on one hand, the losses of material, the material is, on the other hand, subjected to greater time and thermal stresses when passing through the rheometer. In the subsequent optical examination, this may result in the indication of faults, which are not present in the original material, so that the measuring result of this subsequent examination is falsified. Also, U.S. Pat. No. 5,347,852 discloses such an in-line arrangement for the determination of the rheological properties of heated melts. Herein, with a first dosing pump, a melt stream is diverted from a mainstream to be processed. A second dosing pump pumps the diverted stream back into the main stream after it has been conducted through a capillary arrangement in which the pressure and temperature of the diverted melt stream have been determined. In the arrangement, the first and second dosing pumps are controllable independently from each other such that the pressure at the end of the capillary arrangement can be maintained essentially constant.
The arrangements known in the art consequently have many disadvantages, such as:
either a small range of sheared velocities that can be covered;
or a small measuring accuracy in certain ranges,
or long dwell times with the chance of thermal damages;
or high melt consumption resulting in increased expenses;
or negative effects on the optical examination of melt being returned to the main stream.
It is the object of the invention to provide an apparatus and a measuring method by which the above listed disadvantages are avoided. This object is solved by an apparatus and a method according to the invention.
In an apparatus for measuring the viscosity of plastic materials including a heatable housing structure enclosing a controllable material pump for generating a certain material flow, and a plurality of capillaries, a melt distributor is provided in the supply line of the material from the material pump to the capillaries with which the material can be directed to either of the plurality of capillaries and each capillary is provided with a pressure and a material temperature sensor providing the values needed for determining the viscosity of the material.
The apparatus consists essentially of a multi-part rheometer body. The rheometer body is heated from all sides for maintaining an exactly defined melt temperature. The rheometer body includes an inlet passage to which the heated inlet pipe is flanged. The melt is usually prepared in a separate extruder in which it is plasticized and homogenized by being subjected to heat and shear energy. The main stream of the melt is discharged from the extruder by way of a suitable nozzle, for example in the form of a foil, which, after cooling, is utilized for determining the optical quality features of the polymer.
The melt is then supplied by way of a melt channel as a side stream to the gear pump, which is also integrated into the rheometer body. The material which is pre-pressurized by the extruder is supplied to the melt pump, which forces the melt with a constant volume flow through the discharge opening into the melt distributor (melt diverter).
Here, the melt stream is separated into several (2 to n) melt streams, which lead to a melt diverter. The melt diverter is preferably a cylindrical body, which is tightly fitted into the rheometer body. The cylinder includes a suitably arranged bore so that, upon rotation or axial displacement of the cylinder by a predetermined amount, the bore is aligned with another channel for supplying melt to another capillary. This melt diverter is so controlled that a melt stream is conducted only to one of the capillaries, which are firmly installed in the rheometer body downstream of the melt diverter. In front of each capillary, an increased volume pressure-measuring chamber is provided which facilitates the installation of a membrane-type pressure sensor for each of the measuring capillaries.
After flowing through the capillary, the melt exits depressurized and is collected and cooled in a container disposed below.
The individual measuring capillaries are mounted in the rheometer by means of clamping bolts so that they are easily exchangeable.
The measuring procedure is as follows:
The capillaries are so selected that, on one hand, a large shear speed range can be covered (about 1:1000) and, on the other hand, the pressure build-up will not exceed the values admissible for the operation of the melt pump and the pressure sensors. For each measuring range, an optimally suitable pressure sensor is installed.
The pump is set to operate at a certain speed for providing a certain flow volume. The melt diverter is positioned for directing the flow to the first capillary. After a predetermined flushing period, several measurements are performed for determining a reliable average value.
After a certain time, the melt diverter is readjusted so as to direct the melt flow to the next capillary. This process may then be repeated for a third capillary. The measurement at constant throughput of the melt pump provides in this case already three points of a flow curve in the shear velocity range of up to 1:1000 or more. This measuring result is sufficient for most applications in the continuous quality control.
After completion of a measuring series, a computation procedure for the determination of the three measuring points of a flow curve is automatically initiated. Herefrom, for example, the Careau equation, the complete flow curve is calculated and a standard value for the flowability, for example, an MFI-value is determined. In this way, a continuously corrected viscosity value is available in the process control in the form of a curve or as an individual value.
If in the laboratory or in research and development, a larger measuring range or a more accurate determination of the flowability curve is desired, additionally the speed of the melt pump may be changed whereby a range of the shear velocity of more than 104 can be covered.
The advantages of the apparatus according to the invention are therefore as follows:
a wide range of the shear speed of up to 1:1000 is covered already with a constant pump speed and a constant melt stream without affecting the main melt stream of the extruder,
the flow curve can properly be established by the determination of the three different measuring points for the whole shear speed range without the need for changing capillaries by hand and calibrating them;
with a variable pump speed a shear speed range of up to 1:104 can be covered; with the additional control of the pump speed, for example with a second pump speed, six or even eight points of a flow curve can be determined;
the measurement values are highly accurate and reproducible as an optimal pressure sensor is provided for each capillary;
the consumption of polymer material is relatively low since, by using of a melt diverter, the melt material flow can be directed to one capillary after the other; and
the flow curve and a standard fixed value (for example MFI) are determined fully automatically.
The advantages and features of the present invention will become apparent from the following description of embodiments in connection with the drawings.